This invention relates to a unique placement for an unloader valve that is particularly beneficial to a compressor that operates in economized cycle and can also be unloaded through an intermediate economizer port.
One of the compressor types that are especially suited for this invention is a scroll compressor. Scroll compressors are becoming widely utilized in compression applications. However, scroll compressors present several design challenges. One particular design challenge is achieving reduced capacity levels when full capacity operation is not desired.
Thus, scroll compressors, as an example, have been provided with unloader bypass valves that divert a portion of the compressed refrigerant back to a compressor suction port. In this way, the mass of refrigerant being compressed is reduced. Of course, other compressor types may also have a bypass valve for similar purpose.
On the other hand, in many refrigerant compression applications, there are other times when it would be more desirable to have the ability to also achieve increased unit capacity. One way of achieving increased capacity is the inclusion of an economizer circuit into the refrigerant system. An economizer circuit essentially provides heat transfer between a main refrigerant flow downstream of the condenser, and a second refrigerant flow which is also tapped downstream of the condenser and passed through an expansion valve. The main flow is cooled in a heat exchanger by the second flow. In this way, the main flow from the condenser is cooled before passing through its own expansion valve and entering the  evaporator. Since the main flow enters the expansion valve at a cooler temperature, it has greater capacity to absorb heat, and provides increased system cooling capacity, which was the original objective. The refrigerant in the second flow preferably enters the compression chambers at an intermediate compression point, slightly downstream of suction. Typically, the economizer fluid is injected at a point after the compression chambers have been closed.
In a system disclosed in U.S. Pat. No. 5,996,364, a refrigerant system has both a bypass line and an economizer circuit. The bypass line communicates the vapor from intermediate compression point directly to the suction line. This bypass line is provided with the unloader valve. When it is desired to have unloaded operation, the unloader valve is opened, and the economizer valve is closed. Refrigerant may thus then be returned from an intermediate point in the compression cycle directly back to suction.
While this prior art system has achieved many benefits, there are certain additional refinements that would be beneficial.